(1) Field of the Invention
The present invention relates an induction synchronous motor of a brushless type.
(2) Description of the Prior Art
Generally, a synchronous motor requires a starting means for accelerating its rotor to a rotating speed of rotating magnetic fields produced by stator windings, that is, approximately to a synchronous speed, and a means for effecting DC excitation of rotor windings for the normal running operation.
An induction synchronous motor has been devised for omitting a starting means and having the synchronous motor itself posses a starting torque, so that the motor can first start as an induction motor with the rotor windings being short-circuited whereby no special means for starting the motor is required. However, such motor requires brushes since the rotor windings must be excited by DC current for synchronous operation of the motor. When the rotating speed of the rotor approaches the synchronous speed, the short-circuiting of the rotor windings is released so that the DC current is allowed to flow to the rotor windings through the brushes from the external DC current power source thereby producing magnetic poles in the rotor. These magnetic poles are attracted by the rotating magnetic fields produced by the stator windings so that the rotor is caused to be rotated at a synchronous speed. The brushes require maintenance checks resulting in extra costs for maintenance so that the development of a synchronous motor having a brushless configuration is strongly desired.
As conventional synchronous motors having a brushless configuration, there are motors of permanent magnet types or reluctance types but these are all limited to small capacity motors because induction starting is not possible and the starting torque is small. Such motors of an inductor type have disadvantages because magnetic passages therein are complex resulting in an increase in the size of the motors. The same is true in the motors using therein an AC exciter and a rotary rectifier means. A three-phase synchronous motor of a brushless type utilizing a harmonic magnetic field through a square wave voltage of an invertor by having a diode connected to the rotor windings has a disadvantage in that the magnetic magnetizing power of the rotor is insufficient and the output is not strong enough.
Also known is a brushless three-phase synchronous motor of a self-exicitation type. In such motor, a diode is inserted in one phase of three-phase stator windings whereby a static magnetic field is superimposed over a positive phase component rotating magnetic field and an alternating current voltage due to the static magnetic field is induced in the rotor windings to rotate in the synchronous speed. By having the voltage thus induced rectified through the diode, the rotor winding is DC-excited and a positive phase component of magnetic field acts thereby producing a synchronous torque. However, a drawback in such motor is that an induction starting is impossible since the starting is effected by eddy currents and the starting torque itself is small.